1. Field of the Invention
This invention relates generally to disposable injector devices and more specifically, a disposable injector device which does not require a check valve but rather, a self-contained injection capsule and spring aided delivery of medicament into subcutaneous tissue of a patient.
2. Description of the Prior Art
Vaccines and drugs in today""s world, are effective in controlling disease; however, parenteral injections possess serious problems which have continued to persist since the first subcutaneous injection was performed in 1836 by Lafargue. [Aronson J K. Routes of Drug Administration: 7 Subcutaneous administration Prescriber""s Journal 38 50-55 (1988).] For instance, the use of a standard hollow needle attached to a syringe is not only inherently dangerous and cumbersome, but requires thirteen different steps to be completed using accepted sterile techniques.
As early as the 1920s and 1930s, several incidents occurred involving the accidental injection of diesel fuel into the hands of engineers due to pinhole defects in high-pressure fuel lines. Fine streams of liquid under sufficient pressure behaved as a xe2x80x9cliquid nailxe2x80x9d and painlessly penetrated the skin. Since the 1940s, numerous designs for high-pressure liquid jet injectors loosely modeled after the principles of fuel injectors continued to be patented. However, most of the complex designs required precision engineering with dozens of machined parts, therefore, they are inherently expensive and complicated. To a large extent, the complexity was due to a perceived need to maintain high pressure throughout each injection as with the high-pressure fuel injectors. Scherer, on the other hand, realized in 1949 that high pressure was required only at the start of the injection in order to punch a hole or track through the tough epidermis. The remaining bulk of the injection could be subsequently infused along the track under much lower pressure. [Scherer U.S. Pat. No. 2,704,543 (1949)].
Standard high-pressure instruments, because of their complexity, were not considered disposable devices. Even though simplified designs do exist [Alchas et al EP Patent No. EP0595508 (1994)], such devices are still complex, expensive and not considered truly disposable. Instead, design improvements have mostly been directed toward production of robust and reliable heavy-duty machines capable of numerous injections at high rates for mass immunization campaigns. [Ismach U.S. Pat. No. 3,057,349 (1959), Landau U.S. Pat. No. 4,266,541 (1981), U.S. Pat. No. 5,746,714 (1988), D""Antonio et al PCT Patent No. WO98/17322 (1998), Parsons PCT Patent No. WO98/15307 (1988)]. Some of the aforementioned prior art, claim capability of 4,000 injections per hour.
Problems continued to threaten the status of jet injections when an outbreak of hepatitis B caused widespread concern in the vaccine industry. [Canter J; et al 1990. An outbreak of hepatitis B associated with jet injections in a weight reduction clinic. Arch. Intern. Med.; 150: 1923-7]. The transfer of the virus through jet injection and its mechanisms were explained at the World Health Organization (WHO) meetings. [J Lloyd. Status of Jet Injectors. SEE HYPERLINK (http://www.who.int/gpv-coldchain/Powerpoint Technet. htm and http://www.who.int/gpv-coldchain/Powerpoint Technet.htm)].
It appeared that high pressure occurring in the tissues, which were suddenly distended by the injection, coincided with falling pressure inside the jet injector. Ultimately, this caused a reflux flow or xe2x80x9csucking-backxe2x80x9d of tissue fluid into the injector. Because of this serious drawback, single-use vials which insert into the mechanical injector were developed. [Parent du Chxc3xa2telet el al Clinical immunogenicity and tolerance studies of liquid vaccines delivered by jet-injector and a new single-use cartridge (Imule): comparison with standard syringe injection. Imule Investigators Group. Vaccine, 15: 449-58 (1987)]. Cheap, plastic, replaceable nozzles and vaccine fluid paths for mechanical injectors have also been developed. Such injectors retain the multidose vial format as in Landau U.S. Pat. No. 4,966,581 (1990)].
A truly disposable liquid jet injector that operated on the new principle of using modest pressure of the human hand to generate a brief pulse of high pressure has been developed. This punches a narrow track through the skin to allow the subsequent delivery of the bulk of the dose at lower pressure. (Roser, B. Disposable Injector Device U.S. Pat. No. 6,102,896). However, these designs still suffer from several disadvantages. The power derived from steady pressure of the hand, converts to a sharp pulse of high-pressure following the structural failure of xe2x80x9csnap tabsxe2x80x9d or the sudden overcoming of the resistance of xe2x80x9csnap rings.xe2x80x9d The liquid dose to be injected is located in a centrally located reservoir, and the high pressure barrel is located in the base of the injector itself which also has an injection orifice in the base. Such an invention, places demands on manufacture and assembly. Both the liquid reservoir assembly and the injector itself need sterile manufacture; engineering to withstand high pressure pulses; and a valve system to isolate the high pressure pulse from the hand activating the device. This leads to an increase in production costs.
Additionally, the completion of the power stroke is dependent upon the maintenance of hand pressure until the full dose of liquid is delivered. This leaves room for error, namely, it cannot be guaranteed that a reflex arrest of hand pressure could not occur under unusual circumstances, aborting the injection before the designed dose is delivered. A preferred solution to these problems is a self-contained injection capsule which can be driven to complete the power stroke by stored energy, thereby eliminating the need for check valves.
The present invention addresses the aforementioned drawbacks, and it is also cheaper to manufacture and assemble. Furthermore, it is possible to separate the dose capsule and the power assembly, meaning injection kits could contain a single power device and multiple dose capsules. Although superficially similar to the Imule concept [Parent du Chxc3xa2telet et al Clinical immunogenecity and tolerance studies of liquid vaccines delivered by jet-injector and a new single-use cartridge (Imule): comparison with standard syringe injection. Imule Investigators Group. Vaccine, 15: 449-58 (1997)], the design of the separate injection capsule in the present invention is novel and far superior.
Central to this improvement is a narrow-bore element of a plunger design, which forces a small volume of a dose under high-pressure through the skin to provide an injection track, which element forms a part of the low-pressure plunger which subsequently delivers the bulk of the dose. In addition, a fine injection orifice is incorporated into the other end of the injection capsule thereby making the entire liquid path fully self-contained. Essentially, the plunger is a two-component telescopic assembly in which a narrow plunger is centrally located concentrically in a wider plunger of annular shape. The xe2x80x9corifice endxe2x80x9d of the narrow plunger tightly fits in a cylindrical cavity which formed in the central portion of the annular wide plunger. The major portion of the narrow plunger is located further from the injection orifice than the annular wide plunger, thus defining a volume of liquid in the central portions of the annular wide plunger which generates a high-pressure jet during the injection stroke.
To address the problem of lack of guaranteed completion of the power stroke, one solution is to transiently store the energy generated by hand pressure in a spring and releasing the stored energy by means of a snap device. By tensioning the spring only at the time of use, the power stroke is in minimal danger of prematurely aborting the injection. This avoids the need for storage of energy in the device while on the shelf, since doing so can lead to degradation of the material and stored energy over time, thereby reducing the shelf-life of the device.
A spring which is not under tension while stored, whether a conventional metal spring or gas spring, does not undergo material fatigue or energy leakage. Rather, maximum potential energy conferred by hand is delivered as a kinetic force, at the time of use.